Recent studies have shown that median eminence (ME) norepinephrine (NE) turnover rates rise in cyclic rats during the afternoon of proestrus concomitant with the preovulatory LH surge. Similar increases in NE turnover rates in ME have been observed follwing treatment of ovariectomized (ovx) rats with estradiol (E2). However, in the presence of progesterone (P), NE turnover rates are advanced by 2 hours and the LH surge occurs 60 minutes earlier than it does in rats receiving only estrogen. Twenty-four hours following progesterone exposure of estrogen-treated ovx rats, LH surges are extinguished for the next 2-3 days while they continue to occur each afternoon for 4-5 days in ovx rats receiving only estrogen. We plan to examine several CNS molecular processes which may be involved in this "down regulation" of LH surges by progesterone: (1) the effects of progesterone on estrogen nuclear binding in various CNS areas on the day of LH surge and 24 and 48 hours later; (2) the effects of progesterone on reappearance of progesterone cytosol receptor 2-3 days after initial exposure; (3) effects of estrogen alone or with progesterone on ME catecholamine turnover rates on days 2-3 when LH surges do not oocur in E2P-treated versus E2; (4) effects of E2 and E2P on LHRH synthesis in preoptic area, suprachiasmatic nuclei and ME on days 2-3; (5) selective effects of anti-estrogen on LHRH synthesis versus catecholamine turnovers in the endocrine brain.